Merge fast/masking into css3/masking

These two cover the same specification (CSS Masking), so merge the
smaller corpus into the larger one (also preferring non-fast over fast.)

The following tests are converted to testharness.js:

  css3/masking/clip-path-selection.html
  css3/masking/parsing-clip-path-shape.html
  css3/masking/parsing-mask-source-type.html
  css3/masking/parsing-mask.html

and additionally css3/masking/clip-path-selection.html is converted
away from using eventSender.

Bug: 885137
Change-Id: I36607239ec37c69095632fe409f8a315ceb3e31e
Reviewed-on: https://chromium-review.googlesource.com/1230027
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592063}
21 files changed